It all depends how much time you want to invest. I usually find I can do better than the price aggregators (like Kayak etc.) with coupon/discount codes or memberships. Try any airlines you are part of their frequent flyer program for, for example. Also the
AAA specials/rates at Hertz can be pretty good if you are a member. I used to avoid Hertz but now I have to use them when I travel for work and have found their Gold Plus member program to be awesome (free to join) and have been pleasantly surprised that their rates for personal bookings have been amongst the cheapest for me several times recently (especially with AAA discount). MouseSavers usually has a good listing of various promo or discount codes to try for each company as well.
Autoslash basically does all that promo code testing for you but I've never actually used it personally as I didn't want to wait for them to email me back a quote. I do see they also have a listing of promo codes on their website that you can try on your own.
Whoever you end up booking with one of the most annoying things about rental cars is that prices fluctuate wildly. You can often save more $$ by continuing to check rates and rebooking. I think Autoslash will also monitor that for you or you can do it yourself. Also recommend signing up for the loyalty program with whatever company you rent with before you go. Vehicle pick up is SOOOO much faster for most companies when you are a member of their program (which is almost always free to join).